A fixed amount of 125I labelled steroid competes with the steroid to be measured present in the sample or in the calibrator for a fixed amount of antibody sites immobilized on the wall of a polystyrene tube.
Neither extraction nor chromatography is required because of the high specificity of the coated antibodies.
After 3 hours incubation at 37°C, an aspiration step terminates the competition reaction. The tubes are then washed with 3 ml of washing solution and aspirated.
A calibration curve is plotted and the E2 concentrations of the samples are determined by dose interpolation from the calibration curve.